Gas turbine engines are used for a variety of purposes including electric utility and industrial powerplant applications. These gas turbine engines typically comprise two basic configurations. One configuration, identified as a single shaft system, uses a single compressor which is directly coupled to a single turbine and the turbine or compressor is directly coupled to a load. Alternatively, other systems utilize a free power turbine which is positioned downstream of, and is aerodynamically coupled to, the output of a turbine. These systems using a power turbine may incorporate either a single compressor which is connected to a single turbine or multiple compressors, such as a low pressure compressor which is connected to a low pressure turbine and a high pressure compressor connected to a high pressure turbine. However, in systems using a power turbine the power turbine operates independently and rotates freely of the turbine which is connected to the compressor. Each of these two configurations have their respective advantages and disadvantages. For example, in various applications, such as in marine applications it is desirable to have a large amount of stalled torque such that when the system is connected to a non-rotating load, a large amount of torque may be applied. A large quantity of stalled torque may be obtained with systems incorporating a power turbine as the engine may be operating independently of the power turbine and therefore a large amount of torque may be applied to the load through the power turbine even as the power turbine remains stationary. In contrast, single shaft systems have no stalled torque since the load is directly connected to the turbine and the compressor. Therefore, when the load is non-rotating the compressor and turbine will also not rotate and no power may be applied to the load. In these systems, large starter motors are required to rotate the compressor turbine and load in order to start the engine. However, the single shaft system has advantages over systems incorporating a free power turbine such as the performance obtained as ambient temperature increases. It is generally known that when the load is a generator, systems having a power turbine will experience a greater drop in power output than single shaft systems wherein the generator results in the rotor, and therefore the compressor and turbine, rotating at a relatively constant speed. Single shaft systems also are advantageous with respect to sudden decreases in the amount of load. In a single shaft system when the load becomes disconnected, either electrically or mechanically, from the turbine, the turbine will continue to be connected to the compressor and therefore this provides some overspeed protection. However in power turbine systems, when the load becomes disconnected, the power turbine is not connected to any other component which may limit its overspeed, such as a compressor, and therefore the power turbine is much more subject to rapid overspeed which may result in damage to the power turbine and the entire powerplant when critical stresses occur in the power turbine.
Other considerations in the development and selection of a gas turbine engine include cost, reliability and the amount of experience any gas turbine engine system has obtained. Many industrial powerplants have used engines which are derived from aircraft engines and therefore these industrial engines have benefited from much of the aircraft engine's experience and technology. Additionally the use of aircraft derived engines results in relatively lightweight engines which are particularly valuable in some applications. These aircraft engine derived systems have incorporated the use of power turbines which, while having their advantages, also have their respective disadvantages coupled with the increased cost of supplying an additional power turbine. Further, these power turbines are not typically used in the aircraft engines from which these industrial engines are derived and therefore require additional design and system modifications. These advanced aircraft gas turbine engines often utilize multiple compressors with a high pressure compressor attached to a high pressure turbine through a hollow shaft and a low pressure compressor attached to a low pressure turbine through a shaft which extends through the hollow high pressure shaft. In many applications it may be desirable to have the shaft which drives the load positioned upstream of the compressors. For example boilers may be positioned downstream of the engine for generating steam for injection into the engine. However, having the load also positioned downstream of the power turbine provides undesirable required changes to the flowpath so as to provide the air flow to heat the boilers. Unfortunately, in these advanced gas turbine designs with multiple shafts it is impractical to provide an additional shaft which extends from the power turbine through the engine such that the shaft may be connected to a load upstream of the compressors. Additionally, aircraft derivative engines typically require modifications to the turbine so as to match changes in air flow and other changes often include nozzle area changes and modifications of the number of compressor stages. Whenever changes or modifications are require from the existing aircraft engine design this requires design and manufacturing changes which result in increased expense and result in additional reliability considerations corresponding to the use of configurations which have not benefited directly from the reliability, testing and experience obtained from the respective aircraft engine.
Therefore it would be desirable to have a system which avoids the disadvantages of the single shaft and power turbine configurations while retaining many of their respective advantages. Further, it would be desirable to have a system which minimizes modifications required in existing aircraft engine technology.